Why not just plug it in for a few hours and see if the coolant warms up? Of course you'll have to do this cold...... maybe overnight and check it in the morning. I think the heater is an all or nothing type deal so it will either work fine or not at all.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k